U.S. patent number 6,668,006 [Application Number 09/478,717] was granted by the patent office on 2003-12-23 for integrated optical device for data communication.
This patent grant is currently assigned to Lambda Crossing Ltd.. Invention is credited to Moti Margalit, Meir Orenstein.
United States Patent |
6,668,006 |
Margalit , et al. |
December 23, 2003 |
Integrated optical device for data communication
Abstract
An optical device for use in data communication technique is
presented. The device comprises a combination of two spaced-apart
waveguides and at least two spaced-apart resonator-cavity loops.
The resonator-cavity loops are accommodated between the two
waveguides and connected to each other through sections of the
waveguides in such a manner that the resonator-cavity loops and the
waveguide sections create a closed loop compound resonator for
storing optical energy of a predetermined frequency range. A
control means is used for controlling physical characteristics of
the compound resonator to adjust its optical storage
characteristics.
Inventors: |
Margalit; Moti (Zichron Yaagov,
IL), Orenstein; Meir (Haifa, IL) |
Assignee: |
Lambda Crossing Ltd. (Caeseria,
IL)
|
Family
ID: |
11073331 |
Appl.
No.: |
09/478,717 |
Filed: |
January 6, 2000 |
Foreign Application Priority Data
Current U.S.
Class: |
372/97; 372/92;
372/94; 385/2; 385/24 |
Current CPC
Class: |
G02B
6/12007 (20130101); G02F 1/313 (20130101); H01S
5/1071 (20130101); H01S 5/1075 (20130101); G02F
1/3133 (20130101); G02F 2203/055 (20130101); H01S
5/0625 (20130101); H01S 5/10 (20130101); H01S
5/1021 (20130101); H01S 5/1028 (20130101); H01S
5/1032 (20130101) |
Current International
Class: |
G02B
6/34 (20060101); G02F 1/29 (20060101); G02F
1/313 (20060101); G02B 6/12 (20060101); H01S
5/10 (20060101); H01S 5/00 (20060101); H01S
5/0625 (20060101); H01S 003/08 (); H01S 003/083 ();
H01S 003/082 () |
Field of
Search: |
;372/97,92,94
;385/2,24,50 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2118765 |
|
Nov 1983 |
|
GB |
|
9853535 |
|
Nov 1998 |
|
WO |
|
9857207 |
|
Dec 1998 |
|
WO |
|
9917151 |
|
Apr 1999 |
|
WO |
|
Other References
Microring resonator channel dropping filters, Little et al, Journal
of lightwave technology, vol. 15,No. 6, Jun. 1997.* .
Little, B.E., et al. "Microring Resonator Channel Dropping
Filters". Journal of Lightwave Technology, U.S. IEEE, vol. 15, No.
6 (1997) pp 998-1005..
|
Primary Examiner: Ip; Paul
Assistant Examiner: Rodriguez; Armando
Attorney, Agent or Firm: Ladas and Parry
Claims
What is claimed is:
1. An optical device comprising: (a) a combination of two
spaced-apart waveguides and at least two spaced-apart
resonator-cavity loops accommodated in a spaced-apart relationship
between said two waveguides, each of said at least two spaced-apart
resonator cavity loops being optically coupled to the two
waveguides, and said at least two spaced-apart resonator-cavity
loops being optically coupled to each other through sections of
said waveguides in the space between the resonator-cavity loops,
said at least two spaced-apart resonator-cavity loops and said
sections of said waveguides creating a closed loop compound
resonator cavity for storing optical energy of a predetermined
frequency range adjustable by controlling physical characteristics
of the compound resonator; and (b) a control means associated with
either one of the waveguide sections for affecting a change in a
roundtrip phase of the compound resonator cavity, thereby
controlling the physical characteristics of the compound resonator
to adjusts its optical storage characteristics.
2. The device according to claim 1, wherein said control means is
further associated with either one of the resonator cavities.
3. The device according to claim 1 wherein said control means
comprises heating means.
4. The device according to claim 1, wherein said control means
comprises electro-optic means.
5. The device according to claim 1, wherein said control means
comprises piezo-optic means.
6. The device according to claim 1, functioning as a switch.
7. The device according to claim 1, functioning as a modulator.
8. The device according to claim 1, wherein the resonator-cavity
loops and the waveguides are arranged within same optical
layer.
9. The device according to claim 1, wherein the resonator cavity
loops are arranged within same optical layer, and the waveguides
are arranged in a locally adjacent layer.
10. The device according to claim 1, and also comprising at least
one additional pair of resonator-cavity loops, all the resonator
cavity loops being aligned is a line between the two
waveguides.
11. The device according to claim 1, wherein said resonator-cavity
loops are filled with an optically active medium providing optical
gain, the device thereby operating as a laser or amplifier.
12. The device according to claim 1, wherein said waveguide
sections are filled with an optically active medium providing
optical gain, the device thereby operating as a laser or
amplifier.
13. The device according to claim 7, wherein said resonator-cavity
loops are filled with an optically active medium providing optical
loops.
14. The device according to claim 7, wherein said waveguide
sections are filled with an optically active medium providing
optical loss.
15. A controlled laser device comprising: a combination of two
spaced-apart waveguides and at least two spaced-apart
resonator-cavity loops accommodated between the two waveguides and
connected to each other through sections of said waveguides, said
at least two spaced-apart resonator-cavity loops and said waveguide
sections creating a closed loop compound resonator for storing
optical energy of a predetermined frequency range, wherein at least
one of said waveguide sections or said closed loop resonators is
filled with an optically active medium providing gain effect; and a
control means associated at least with either one of the waveguide
sections for affecting a change in a roundtrip phase of the
compound resonator cavity, thereby controlling the physical
characteristics of the compound resonator to adjust its optical
storage characteristics.
16. The device according to claim 15, wherein said control means is
used to Q-switch said controlled laser.
17. A waveguide router system comprising at least one optical
switch and at least one optical filter connected to an output of
said at least one optical switch, wherein each of the switch and
the filter comprises a combination of two spaced-apart waveguides
and at least two spaced-apart resonator-cavity loops accommodated
between the two waveguides and connected to each other through
sections of the waveguides, the resonator cavities and the
waveguide sections creating a closed loop compound resonator for
storing optical energy of a predetermined frequency range, a
control means associated at least with either one of the waveguide
sections for affecting a change in a roundtrip phase of the
compound resonator cavity, thereby controlling the physical
characteristics of the compound resonator to adjust its optical
storage characteristics.
18. An integrated electro-optical device comprising: a combination
of two spaced-apart waveguides and at least two spaced-apart
wavelength-selective elements accommodated between the two
waveguides and connected to each other through sections of the
waveguides, said at least two spaced-apart wavelength-selective
elements and said waveguide sections creating a closed loop
compound resonator for storing optical energy of a predetermined
frequency range; and a control means associated at least with
either one of the waveguide sections for affecting a change in a
roundtrip phase of the compound resonator cavity, thereby
controlling the physical characteristics of the compound resonator
to adjust its optical storage characteristics.
19. The device according to claim 1, wherein said compound
resonator is a multi-layer structure, the waveguides and the
resonator-cavity loops being located in two different layers,
respectively, of the multi-layer structure.
20. An optical device comprising: (a) a combination of two
spaced-apart waveguides and at least two resonator-cavity loops
accommodated in a spaced-apart relationship between said two
waveguides, each of said at least two spaced-apart resonator cavity
loops being optically coupled to the two waveguides, and said at
least two spaced-apart resonator-cavity loops being optically
coupled to each other through sections of said waveguides in the
space between the resonator-cavity loops, said at least two
spaced-apart resonator-cavity loops and said sections of said
waveguides creating a closed loop compound resonator cavity for
storing optical energy of a predetermined frequency range
adjustable by controlling physical characteristics of the compound
resonator (b) at least one structure composed of an additional
resonator-cavity loop optically coupled to said compound resonator
through one of said two waveguides, and an additional waveguide
coupled to said additional resonator-cavity loop; and (c) a control
means for controlling the physical characteristics of the compound
resonator to adjust its optical storage characteristics.
21. An optical device comprising: (a) a combination of two
spaced-apart waveguides and at least two resonator-cavity loops
accommodated in a spaced-apart relationship between said two
waveguides, each of said at least two spaced-apart resonator cavity
loops being optically coupled to the two waveguides, and said at
least two spaced-apart resonator-cavity loops being optically
coupled to each other through sections of said waveguides in the
space between the resonator-cavity loops, said at least two
spaced-apart resonator-cavity loops and said sections of said
waveguides creating a closed loop compound resonator cavity for
storing optical energy of a predetermined frequency range
adjustable by controlling physical characteristics of the compound
resonator; (b) a plurality of structures each composed of an
additional resonator-cavity loop optically coupled to said compound
resonator through one of said two waveguides, and an additional
waveguide coupled to said additional resonator-cavity loop,
operable to form together a plurality of frequency selective
switches, thereby providing for complex optical signal switching
and routing; and (c) a control means for controlling the physical
characteristics of the compound resonator to adjust its optical
storage characteristics.
22. A controlled laser device comprising: a combination of two
spaced-apart waveguides and at least two spaced-apart
resonator-cavity loops accommodated between the two waveguides and
connected to each other through sections of said waveguides, said
at least two spaced-apart resonator-cavity loops and said waveguide
sections creating a closed loop compound resonator for storing
optical energy of a predetermined frequency range, wherein at least
one of said waveguide sections or said closed loop resonators is
filled with an optically active medium providing gain effect; and a
control means for controlling the physical characteristics of the
compound resonator to adjust its optical storage characteristics,
said control means operating for passively or actively mode locking
of said controlled laser.
23. A wavelength router system comprising at least one switch and
at least one optical filter connected to an output of said at least
one optical switch, wherein each of the switch and the filter
comprises a combination of two spaced-apart waveguides and at least
two spaced-apart resonator-cavity loops accommodated between the
two waveguides and connected to each other through sections of the
waveguides, the resonator cavities and the waveguide sections
creating a closed loop compound resonator for storing optical
energy of a predetermined frequency range; a control means for
controlling physical characteristics of the composed resonator to
adjust its optical storage characteristics; and a control unit
including at least one photodetector connected to the output of the
switch for monitoring optical power through the switch.
24. An electro-optical device comprising: a combination of two
spaced apart waveguides and at least two spaced-apart grating
elements accommodated between the two waveguides and connected to
each other through sections of the waveguides, said at least two
spaced-apart grating elements and said waveguide sections creating
at closed loop compound resonator for storing optical energy of a
predetermined frequency range; and a control means for controlling
physical characteristics of the compound resonator to adjust its
optical storage characteristics.
Description
FIELD OF THE INVENTION
This invention is generally in the field of optical communication
techniques, and relates to an integrated optical device
particularly enabling frequency selection, switching, modulation
and signal routing.
BACKGROUND OF THE INVENTION
Optical communication is the enabling technology for the
information age, and the essential backbone for long haul
communication. As this technology progresses, there is a tremendous
interest in providing optical routes in the short haul,
metropolitan and access networks, as well as in local area networks
and cable TV networks. In all these networks, the best of breed
solution for bandwidth expansion has been the adoption of
wavelength division multiplexing (WDM), which entails the
aggregation of many different information carrying light streams on
the same optical fiber. A device capable of accessing an individual
information stream is fundamentally required in current and future
networks. These devices can also add information streams to the
optical fiber, as well as impress information on an optical stream
by optical modulation.
The basic building block for optical switching is the optical
modulator/switch. Various implementations of such a device have
been developed, of which the most dominant is the Mach-Zender
Interferometer (MZI), in which interference is produced between
phase coherent light waves that have traveled over different path
lengths. The basic construction of MZI is schematically illustrated
in FIG. 1. Light input to a modulator 1 is via a single-mode
waveguide. A beam splitter divides the light into two equal beams
that travel through guides 2A and 2B, respectively. By applying
voltage to electrodes 4, the effective path lengths can be varied.
Hence, the optical switching is achieved by creating a phase
difference between two arms of the device (guides 2A and 2B), and
controlling the optical power at the device output.
In general, the performance criteria for the operation of the
wavelength routing elements include: the following: (1) Modulation
depth or contrast ratio, which signifies the ratio between two
("ON" and "OFF") or more states of a switch device; (2) Crosstalk,
which defines the ability of the device to select a single optical
channel while suppressing information from the other channels; (3)
Electric power consumption; (4) Modulation bandwidth, which defines
the speed at which the switching can be achieved; and (5) Optical
bandwidth within which the modulation is effective.
To achieve a good modulation performance with the MZI, the latter
is typically designed with long interference arms. As a result,
this device is not efficient in its implementation, and limits the
scaling ability of complex optical circuits. Another drawback of
MZI-type devices, in their predominant implementation, is their
frequency insensitiveness over a desired frequency bandwidth. As a
result, MZI-type devices cannot be used directly for wavelength
routing.
To achieve wavelength routing, the MZI has been utilized in
conjunction with wavelength demultiplexers, which provide spatial
separation between different optical frequencies. To this end, a
matrix composed of at least N times (N+1) MZI is used to redirect
one of the N spatially distinct wavelengths to the device output.
The remaining frequencies are recombined using a wavelength
multiplexer.
Recently developed integrated electro-optical devices utilize
resonant rings to achieve frequency selective switching. Such a
device is disclosed, for example, in WO 99/17151. The main
components of the device are illustrated in FIG. 2. A resonant ring
6 couples light from one fiber 8a to another fiber 8b, when the
frequency of the light passing through the fiber 8a fulfils that of
the resonance condition of the ring 6. By applying an electric
field or a thermal source to the ring 6, its refractive index, and
consequently, its resonance condition, can be desirably adjusted.
Changing the resonance condition prevents the passage of the
previously coupled light and acts as a switch. Alternatively, the
loss of the ring waveguide can be changed. Adding loss to the ring
diminishes its operation as a resonant cavity, and light cannot be
coupled from fiber to fiber.
Unfortunately, the conventional resonant ring based systems require
fabrication tolerances that are hard to implement by means of a
conventional photolithography technique. This disadvantage becomes
more essential in multiple-ring devices, wherein the distance
between two locally adjacent rings is a critical factor for the
successful operation of the device. The use of a switching
mechanism providing de-tuning of a resonant ring out of resonance
condition has been proposed, being disclosed for example in WO
98/53535. This solution, however, does not meet the extinction
ratio and crosstalk requirements of communication systems.
SUMMARY OF THE INVENTION
There is accordingly a need in the art to improve the operation of
electro-optical communication devices by providing a novel
electro-optical device such as an optical frequency dependent
switch and a modulator.
The present invention takes advantage of the use of several (at
least two) ring resonators. The main idea of the present invention
is based on designing an optical complex filter/resonator, wherein
waveguide sections are specifically connected to ring resonators in
a configuration which enables realization of optical switching,
wavelength routing, lasing, wavelength sensitive amplification and
optical filtering. The device may also combine a plurality of such
filters in a wavelength router module. Generally speaking, the
present invention utilizes the collective response of two or more
closed loop resonators, which are connected to each other by two or
more optical paths, for the purpose of switching or modulating a
selected wavelength.
The fabrication technology of waveguides is well developed in many
classes and families of materials. The relaxation of the
fabrication tolerances in the present invention relates to the
possibility of vertical coupling of light from the waveguides to
the ring resonators. Since the vertical fabrication tolerances are
much better then the horizontal tolerances, the result is a device
which is simpler to manufacture. However the details and design of
the invention extend beyond such devices in which only vertical
coupling exists between the waveguides and resonators.
The optical resonator according to the invention is an enclosed
cavity aimed at storing optical energy. As compared to the known
devices of the kind specified, which utilize a closed-loop type
optical resonator, and several such resonators cascaded in various
configurations, the present invention utilizes the inclusion of a
feedback path for the optical signal. In other words, in the
present invention, the loop resonator serves as a frequency
selective mirror within a more complex resonator. These mirrors
together with connecting waveguides create closed loop cavities
with superior performance and simpler fabrication, and, mainly, are
favorable for inclusion of switching capabilities and active media,
as compared to the conventional devices.
The wavelength response of a structure composed of several ring
resonators coupled to optical waveguides is determined by the
physical and geometrical parameters of the resonators and coupling
scheme. The present invention provides novel schemes of coupling
multiple resonators to achieve predetermined active filtering and
modulation characteristics. These coupling schemes are relatively
easy to implement, and provide desired modulation
characteristics.
There is thus provided, according to one aspect of the present
invention, an optical device comprising: (a) a combination of two
spaced-apart waveguides and at least two spaced-apart
resonator-cavity loops accommodated between the two waveguides and
connected to each other through sections of the waveguides, said at
least two spaced-apart resonator-cavity loops and said waveguide
sections creating a closed loop compound resonator for storing
optical energy of a predetermined frequency range; and (b) a
control means for controlling physical characteristics of the
compound resonator to adjust its optical storage
characteristics.
The predetermined frequency range is determined by physical and
geometrical characteristics of the compound resonator. To control
the physical characteristics of the waveguide and/or
loop-resonators, a heating means may be used.
One of the two waveguides serves as an input and throughput
waveguide, and the other serves as an output waveguide. An optical
signal entering the input waveguide may include a plurality of
light components having different wavelengths. By actively
adjusting the response of the compound resonator, using a heater or
any other means that changes the characteristics of the waveguide
sections, one of these wavelengths may be switched from the input
to the output waveguide.
The device may comprise additional waveguides and additional
loop-resonators, forming together several such frequency selective
switches, thereby providing complex optical signal switching and
routing.
According to another aspect of the present invention, there is
provided a wavelength router system comprising at least one optical
switch and at least one optical filter, wherein the switch and the
filter is constructed as the above-described integrated
electro-optical device.
According to yet another aspect of the present invention, there is
provided a laser device where an active material with gain is
embedded in at least one of the parts comprising the
above-described integrated electro-optical device. This multiple
section laser can be controlled by applying the above-described
control means to tune its lasing frequency, to q-switch or to
passively/actively mode lock the laser device in order to obtain
pulsed operation.
In general, the resonator-cavity loops (ring-resonators) can be
replaced by lo any other implementation of a frequency-selective
element that couple between the two waveguide sections. For
example, optical gratings can be used.
Thus, according to yet another aspect of the present invention,
there is provided an integrated electro-optical device comprising:
a combination of two spaced-apart waveguides and at least two
spaced-apart wavelength-selective elements accommodated between the
two waveguides and connected to each other through sections of the
waveguides, said at least two spaced-apart wavelength-selective
elements and said waveguide sections creating a closed loop
compound resonator for storing optical energy of a predetermined
frequency range; and a control means for controlling physical
characteristics of the compound resonator to adjust its optical
storage characteristics.
According to yet another aspect of the present invention there is
provided a method for manufacturing the above device utilizing
existing lithography techniques.
More specifically, the present invention is used with the
ring-resonators and is therefore described below with respect to
this application.
BRIEF DESCRIPTION OF THE DRAWINGS
In order to understand the invention and to see how it may be
carried out in practice, a preferred embodiment will now be
described, by way of a non-limiting example only, with reference to
the accompanying drawings, in which:
FIG. 1 is a schematic illustration of the MZI structure;
FIG. 2 is a schematic illustration of the conventional resonant
ring based electro-optical device;
FIG. 3 is a schematic illustration of an electro-optical device
according to one embodiment of the invention;
FIG. 4 graphically illustrates some advantageous features of the
device of FIG. 2, as compared to the conventional devices;
FIG. 5 graphically illustrates simulation results of the operation
of the device of FIG. 2;
FIGS. 6a to 6c schematically illustrate electro-optical devices
according to three different embodiments of the invention,
respectively, suitable for designing complex filter structures;
FIGS. 7a to 7c illustrate three more examples, respectively, of
complex filter structures constructed according to the
invention;
FIG. 8 graphically illustrates the operational principles of the
devices of FIGS. 7a-7c;
FIG. 9 schematically illustrates a block diagram of a wavelength
router system utilizing the devices according to the invention;
and
FIG. 10 illustrates a system utilizing the optical switches and
filters according to the invention, and using ASE for monitoring
the status of the optical switches.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
FIGS. 1 and 2 illustrate conventional MZI-type and ring-resonator
type structures, respectively.
Referring to FIGS. 3a and 3b, there is illustrated an
electro-optical device, generally designated 10, constructed and
operated according to one embodiment of the invention. The device
10 includes a compound resonator, which, according to the present
example, is composed of two ring-resonators 12A and 12B
(constituting resonator-cavity loops functioning as
frequency-selective elements), and two waveguides 14 and 16,
wherein waveguide sections 14A and 16A connects the rings 12A and
12B to each other. As shown in FIG. 3b by way of block-diagram,
these waveguide sections 14A and 16A present a spacer S between
optical cavities defined by two ring-resonators R.
Further provided in the device 10 is a heater element 18
(electrodes) placed on either one of the waveguide sections--the
section 16A in the present example. The operation of the heater
element 18 enables to control the refractive index and
consequently, the optical phase imparted by the waveguide spacer S.
Generally speaking, the change in the refractive index will induce
the required phase shift to change the frequency response of the
compound resonator. Such an active phase affecting may be achieved
by applying any suitable thermo-optic, piezo-electric,
electro-optic or the like effects within the spacer or ring
resonator regions.
The device 10 may be implemented as a multi-layer optical structure
manufactured by a lithography technique. All the elements of the
compound resonator, i.e., the ring-resonators and the waveguides,
may be formed in the same optical layer. Alternatively, the
compound resonator may be manufactured as a multi-layer structure,
namely the waveguides may be located in a locally adjacent upper or
lower layer with respect to the layer containing the
ring-resonators. This facilitates the manufacture to meet the
requirements for small spaces between the coupling elements (i.e.,
ring and waveguide).
The optical cavities (ring-resonators) are weakly coupled to the
waveguides. Direct coupling between the two resonators is not
required by this scheme. Each of the optical cavities is capable of
supporting several resonance frequencies, which are determined by
the geometrical and material details of the cavity. In the present
example of FIGS. 3a and 3b, which is the simplest case utilizing a
pair of ring-resonator cavities, the two cavities 12A and 12B are
identical, namely tuned for the same frequency range of the
resonance condition. A change in the refractive index of one of
compound resonator's elements, which is the waveguide 16 in the
present example, since the heating element 18 is associated with
this waveguide), will cause changes in the roundtrip phase of the
entire cavity, thereby shifting the resonance condition.
As indicated above, although the ring-resonators are exemplified
here, any other implementation of the frequency-selective elements
(mirrors) that couple between the two waveguide sections may be
employed in the compound resonator for the purposes of the present
invention. Such a frequency-selective element may, for example, be
an optical gratings.
Assuming that the waveguide 14 is an input waveguide, and the
waveguide 16 is an output waveguide, the response of the compound
resonator 10 is essentially different from that of the conventional
single-ring resonator shown in FIG. 2. This is illustrated in FIG.
4, showing two graphs G.sub.1 and G.sub.2, presenting optical power
P at the output waveguide (fiber) 16 as a function of a normalized
wavelength .lambda..sub.nor, corresponding, respectively, to the
conventional device and device 10 constructed according to the
invention. It is self evident that the filtering characteristics
and out of band signal suppression of the coupled resonator 10 are
much better then those of the conventional single-ring
resonator.
FIG. 5 illustrates simulation results of the operation of the
device 10. Three graphs H.sub.1, H.sub.2 and H.sub.3 presenting the
output power P of the device 10 as a function of normalized
wavelength .lambda..sub.nor for different phase shifts: .phi.=0,
.phi.=.pi./4 and .phi.=.pi./2, respectively.
Each of the ring-resonators (two in the present example) is
comprised of a waveguide with the index or refraction (n.sub.core)
larger then its surrounding material (n.sub.cladding). The
waveguide is fashioned into a closed path, called "ring". The input
waveguide passes below the ring-resonators in a manner to allow the
overlap of the two waveguide modes and allow for transferring
optical power from the input waveguide to the ring-resonator. As
for the output waveguide, it is also placed so as to attain the
coupling to the ring. This output waveguide may serve as the output
of the selected frequency. Alternatively, when the device 10 is to
be used as a modulator, this waveguide can also serve as a
throughput port for optical frequencies different from that of the
modulated signal.
The operation of the device 10 is characterized by the low loss
propagation of the optical mode in the ring waveguide. This is
achieved by utilizing a refractive index contrast between the
waveguide and surrounding material. The ring may be composed of
optical glass with a refractive index of about 1.6-1.9, may be made
from silicon (refraction index of 3.5) or a layered-structure made
of suitable materials such as used in Vertical Emitting Cavity
Lasers (VECSELs). It is known that the ring itself manifests on
frequencies corresponding to its resonance condition. The resonant
frequency of the ring, f0, is given by: ##EQU1##
wherein R is the ring radius measured from the center of the ring
to the center region of the ring waveguide; n.sub.ef is the
effective refraction index of the ring waveguide; M is an integer
value; and c is the speed of light propagation in vacuum. The
effective refraction index can be determined by various known
techniques.
One of the important parameters defining the characteristics of the
entire device 10, is the coupling between the ring and the
waveguide, since it determines the optical bandwidth and photon
lifetime, and, consequently, the modulation efficiency. The power
exchange between the waveguide and the ring is denoted by k.sup.2,
and can be calculated by computing the overlap integral of the
modes of the ring and waveguide multiplied by the interaction
length. The optical bandwidth, .DELTA.f, is then determined as
follows: ##EQU2##
As seen in FIG. 3b, the individual ring-resonator actually presents
a two-port device. The throughput function describing the ring
optical amplitude characteristics is given by: ##EQU3##
while the drop function is given by: ##EQU4##
Here, .omega. is the radial frequency given by: .omega.=2f.
The matrix describing one ring is given by: ##EQU5##
Complex structures can be obtained by multiplying the matrixes of
the corresponding sections. This calculation technique is known per
se, and is used in analyzing complex distributed feedback
lasers.
Turning back to FIG. 3a, it should be understood that, if at least
one of the frequency-selective elements (ring resonators) 12A or
12B or at least one of the waveguide sections 14A or 16A is filled
with an active medium, the device 10 can operate as a laser.
As known, an optical communication system typically requires
stringent switching and routing. A single resonant ring device
usually cannot meet these requirements. To this end, according to
the present invention, filters are designed either as single stage
coupled compound resonators, or as multiple-stage coupled compound
resonators. Compound resonators of such filtering devices are
illustrated in FIGS. 6a-6c, being designated 20, 30 and 40,
respectively. Each of these devices utilizes the compound resonator
structure 10 of FIG. 3a as a frequency selective switch/modulator,
in which the ring-resonators R are coupled both in the forward and
back directions, thereby increasing the degrees of freedom in the
design of filters. It should be understood that, in all the
examples, the matrix model is used in the synthesis and analysis of
the filter/switch/modulator characteristics.
In the example of FIG. 6a, the entire waveguide cavity is formed by
waveguides 14 and 16, and three pairs of ring-resonators R.sub.1
-R.sub.2, R.sub.3 -R.sub.4 and R.sub.5 -R.sub.6 enclosed
therebetween. In the example of FIG. 6b, a multi-stage coupled
compound resonator 30 is composed of two pairs of rings R.sub.1
-R.sub.2 and R.sub.3 -R.sub.4 enclosed between waveguides W.sub.1
and W.sub.2, and an additional ring-resonator R.sub.5 coupled to an
additional waveguide W.sub.3. A multi-stage coupled compound
resonator 40 (FIG. 6c) comprises two compound resonators 10, and
two additional ring-resonators R.sub.5 and R.sub.6, the latter
being is coupled to an output waveguide W.sub.4. By appropriately
adjusting the index of the waveguide in the corresponding compound
resonator, the desired wavelength may be switched from the input to
the output waveguide.
Reference is now made to FIGS. 7a-7c and 8, illustrating the main
constructional and operational principles of three other devices
50, 60 and 70, respectively, that are capable of operating as a
switch or modulator. The devices 50, 60 and 70 have somewhat
different design of waveguides and rings arrangement, as compared
to the previously described examples, as illustrated in the figures
in a self-explanatory manner using same reference number to
identify those components, which are common in all examples. FIG. 8
Illustrates three graphs D.sub.1 D.sub.2 D.sub.3, corresponding to
simulation results of the operation of the devices 50, 60 and 70,
respectively. Each graph presents the optical power P at the output
fiber (W.sub.2 W.sub.3 W.sub.4, respectively) as a function of a
phase shift (p in the waveguide section. As clearly seen in the
figure, for very small values of the phase shift .phi., more than
20 dB of signal extinction is obtained. This enables the size
required by the waveguide sections to be significantly reduced,
since the optical phase shift is accumulated over the length of the
waveguide.
The advantages of the device according to the invention are thus
self-evident. The device attains attractive modulation
characteristics, requires very small phase shifts, and,
consequently, the interaction region, as well as the switching
power, can be minimized. The extinction ratio of the optical signal
meets optical communication standards.
FIG. 9 illustrates a block diagram of a system 100 utilizing the
above-described devices to form a wavelength router. The system
comprises three switches SW.sub.1, SW.sub.2 and SW.sub.3, and two
filter units FU.sub.1 and FU.sub.2. Each of the filter units is
accommodated between two locally adjacent switches, and is designed
so as to, when being actuated, route a specific optical frequency.
In this example, one of the filters is activated at a time, thereby
enabling the routed wavelength to be dynamically chosen. Obviously,
a plurality of switching mechanisms can be used to increase the
number of drop ports. It is important to note that this technique
requires a considerably lower number of switches than that of the
MZI switching matrix. Indeed, for an N-channel, M-drop system, the
MZI switching matrix would require at least (N+M) by N matrix,
while the system according to the present invention would require N
switches with an M by M matrix.
The present invention can also be used for actively monitoring the
switch performance. As is known, on of the crucial issues in modern
communication systems is the status of the on-line switches. A
non-operative switch in either the "ON" or "OFF" position can
degrade the performance of the communication network. Modern
communication systems utilize an erbium-doped fiber to compensate
for loses in the optical fiber, connectors and devices. The
amplifier emits amplified spontaneous emission (ASE) in all optical
frequencies, which are of interest.
In the present invention, it is proposed to use an ASE for
monitoring the status of the optical switches. This concept is
illustrated in FIG. 10, showing a system 200 that utilizes the
components of the above-described system 100, and two
photodetectors PD.sub.1 and PD.sub.2. Each photodetector is placed
at the output of the corresponding switch and is coupled to a
control unit (monitor) CU that monitors the optical power though
this switch. Since the ASE exists at all frequencies, it can be
used to monitor and control the switches.
Those skilled in the art will readily appreciate that various
modifications and changes can be applied to the preferred
embodiment of the present invention as herein before exemplified
without departing from its scope defined in and by the appended
claims. In the method claim which follow, characters which are used
to designate claim steps, are provided for convenience only and do
not apply any particular order of performing the steps.
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